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Video transcript

today we'll be talking about column chromatography what is this even useful for while when drug companies are trying to produce large amounts of Medicine they need to be able to use a purification process that can be done in a pretty large scale so sometimes in their product they need to get just the final active ingredient purified and column chromatography is a great way to do that so how would we set that up in our organic chemistry lab remember that the stationary phase is the silica gel or other material inside the column and the mobile phase is the solvent but you pour into it well let's take a look at the equipment shown here in blue we have the column this has a few different parts has an opening at the top which you can pour things in a stop clock at the bottom which I've currently shown in the closed position and also a flask at the bottom to collect whatever it is you're using but it doesn't have to be a flask you could use test tubes or really any other piece of glassware you'd like so how do we begin first we need to pack this column full of some kind of filtration material but we also want to make sure that that stuff doesn't just run through and spill into the flask so first you want to put a little cotton ball at the very bottom this is very small and usually what you end up doing is taking a long stick and just kind of ramming it right up against the stopcock next what I'd put in is a fine layer of sand and you want to try to get this to be as just horizontal as possible not tilted or slanted and I'll explain that later on but you want to do that with any layer you're adding on to the column they should all be perpendicular to your column next what you do is add in the silica the silica will take up most of your column and you would be using that to fill almost all of it just pouring it all in until it reaches a line of say about here lastly you pour your solvent into the column and make sure that the column is kept wet at all times as if it runs dry and cracks it can cause running and mixing of bands so you would have your solvent line say at about here because if your column dries out it can crack and what you'll see is that this is actually going through again as I said the whole column but this looks a little bit messy so let me just clean that up for a minute next what we'll want to do is load the column with the actual product that we're using now how do we do that you can actually drop it in with a pipette because you want to make sure that the layer is very even so let's try it out here on top you have this fine layer that you're dropping in on top of the silica gel but how can I push this through the column and into the flask well the first step I'll need to take is actually just opening up your stop clock so why do we need to open up the SAP [ __ ] well when you pour in the solvent you want the band to start going down the column and traveling down and that'll only happen if you know the liquid can flow out the other end so let's redraw our silica line in here this is now where your silica is but as you see the original green band has separated into two distinct bands now to kind of see a yellow one and a blue one in real life the colors might not be quite this distinct but you get the idea and as this proceeds again you'll see the silica line but the separation between the two bands will actually become more and more distinct and so far what you've been collecting in your flask is mostly all just solvent but how do we actually collect the whatever's in the blue band I mean we might not know exactly what compound it is but you can tell that by the fact that they're travel travelling at different rates the blue and yellow bands probably have different polarities and our different compounds so when you see that the blue band is getting really close to the bottom they'll want to quickly switch out your old flask for a new one so that you've collected in the new flask is just the Slayer of the blue compound while in your column you still have the yellow layer note that each flask you collect is considered a fraction and that's how you'd conduct column chromatography so earlier I was telling you you want to make sure that these are pretty much horizontal but what happens if instead when you're packing your column we're pouring something in it ends up looking kind of crooked so in this case let's say that our column was pretty crooked what you would see instead this is silica wine but as those two bands traveled through instead of seeing them just parallel to one another and again perfectly perpendicular to your column what you'd see is something that looks more like this have the yellow band kind of slant to it then you'd also have the green band slanted if you had loaded in your compound in the slanted manner and the issue with this is that at a certain time point say if you're trying to collect the fraction that falls between here and here you're not really getting the pure yellow compound or the pure green compound instead you're getting some mixture of the two which shows that this isn't a very efficient purification so what you'll want to do next time is be very careful in the initial stages because a lot of the work would call them chromatography is making sure that you prep it just right and the rest is just waiting and letting your solvent run through so let's review what we learned today we learned how to pack a column using cotton sand and silica gel and we also learned how to separate compounds using column chromatography based on their polarities